Human caliciviruses (HuCVs) are important viral pathogens associated with acute epidemic gastroenteritis and hepatitis in humans. HuCVs include three genogroups: Norwalk virus (NV), Sapporo and Hepatitis E genogroups. The NV genogroup can be divided further into NV-like and Snow Mountain agent (SMA)-like HuCVs. The application of molecular techniques has enabled rapid progress of HuCV investigations. We now recognize that distinct genogroups within Caliciviridae are associated with distinct genomic organization. We know circulating HuCVs are genetically and antigenically diverse. We also know uncharacterized genetically and antigenically distinct strains exist. The new data generated in the past six years have changed our understanding. of HuCVs. We have generated unique reagents, assays, and databases that are critical for further study of HuCVs. We have accumulated a large collection of clinical and field specimens that allow us to perform epidemiology and immunology studies of HuCVs. More importantly, we have a strong background in molecular virology and epidemiology of HuCVs and other enteric gastroenteritis viruses. In this project, we will continue to use the reagents and resources to pursue further understanding of the immunology, epidemiology and molecular virology of HuCVs. Three specific aims are proposed as follows. Specific aim 1. To characterize HuCV genetic and antigenic diversity by cloning and sequencing currently known strains, including new strains, using RT-PCR and new primers. Specific aim 2. To study gene expression and attempt to cultivate HuCVs in cell culture by transfection with viral cDNA or RNA and by infection of cells with virions from stool specimens. Specific aim 3. To characterize the natural history of infection in children and experimentally infected volunteers and to correlate immunity with protection against HuCV infection and illness.